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<h2><b>maskout</b></h2><p>

<code>maskout(<i>expr,mask</i>)</code><p> 

Wherever the <code><i>mask</i></code> values are less than zero, the
values in <code><i>expr</i></code> are set to the missing data value. <p>

Works with gridded or station data. Where <code><i>mask</i></code> values
are positive, the <code><i>expr</i></code> values are not modified. Thus
the result of <code>maskout</code> is data with a possibly increased
number of missing data values. The <code>maskout</code> function, in spite
of its apparant simplicity, is extremely useful.  <p>

<H3>Usage Notes</H3><P>

<h3>Examples</h3><p> 

<ol>
<li>See the Examples for the <a
href="gradfuncconst.html"><code>const</code></a> function for a
description
of using <code>maskout</code> to calculate the percentage of the globe
covered by precipitation. <p>

<li>The <code>maskout</code> function can be used to cause part of the
data to be ignored while doing another calculation. For example, if we
have a land-sea mask, where sea values are negative, and we want to take
some areal average of a quantity only over land: <p>

<dd><code>d aave(maskout(p,mask.2),lon=0,lon=360,lat=0,lat=90) </code><p>

<li>People frequently have trouble using a mask grid, because it is often
put into a seperate file, and given some arbitrary date/time and level.
Thus, it is often necessary to <i>locally override</i> the dimension
environment while using the mask grid: <p>

<dd><code>d aave(maskout(p,mask.2(t=1)),lon=0,lon=360,lat=0,lat=90)
</code><p>

would probably be how Example 2 would have to be expressed in order to
work, with the local override of <code>t=1</code> specified on the mask
data. See the documentation on how GrADS evaluates expressions within the
dimension environment for more information. 
